Integration of chemical catalysis with extractive fermentation to produce fuels.

Nearly one hundred years ago, the fermentative production of acetone by Clostridium acetobutylicum provided a crucial alternative source of this solvent for manufacture of the explosive cordite. Today there is a resurgence of interest in solventogenic Clostridium species to produce n-butanol and ethanol for use as renewable alternative transportation fuels. Acetone, a product of acetone-n-butanol-ethanol (ABE) fermentation, harbours a nucleophilic α-carbon, which is amenable to C-C bond formation with the electrophilic alcohols produced in ABE fermentation. This functionality can be used to form higher-molecular-mass hydrocarbons similar to those found in current jet and diesel fuels. Here we describe the integration of biological and chemocatalytic routes to convert ABE fermentation products efficiently into ketones by a palladium-catalysed alkylation. Tuning of the reaction conditions permits the production of either petrol or jet and diesel precursors. Glyceryl tributyrate was used for the in situ selective extraction of both acetone and alcohols to enable the simple integration of ABE fermentation and chemical catalysis, while reducing the energy demand of the overall process. This process provides a means to selectively produce petrol, jet and diesel blend stocks from lignocellulosic and cane sugars at yields near their theoretical maxima.

Development of dietary soluble fibres by enzymatic synthesis and assessment of their digestibility in in vitro, animal and randomised clinical trial models.

The aim was to develop novel fibres by enzymatic synthesis, to determine their total dietary fibre by AOAC method 2009.01 and to estimate their potential digestibility and assess their digestibility in vivo using glycaemic and insulinaemic responses as markers in mice and randomised clinical trial models. We found that fibre candidates to which α-(1,2) branching was added were resistant to digestion in the mouse model, depending on the amount of branching. These results show that in vivo models are needed to reliably assess the digestibility of α-glycosidic-linked oligomeric dietary fibre candidates, possibly due to absence of brush border α-glucosidase activity in the current in vitro assessment. α-(1,3)-linked and α-(1,6)-linked glucose oligomers were completely digested in humans and mice. In conclusion, it is possible to develop dietary soluble fibres by enzymatic synthesis. Adding α-(1,2) branching increases their resistance to digestion in vivo and can thus improve their suitability as potential fibre candidates. Clinical Trial Registry: ClinicalTrials.gov, NCT02701270.

Co-production of acetone and ethanol with molar ratio control enables production of improved gasoline or jet fuel blends.

The fermentation of simple sugars to ethanol has been the most successful biofuel process to displace fossil fuel consumption worldwide thus far. However, the physical properties of ethanol and automotive components limit its application in most cases to 10-15 vol% blends with conventional gasoline. Fermentative co-production of ethanol and acetone coupled with a catalytic alkylation reaction could enable the production of gasoline blendstocks enriched in higher-chain oxygenates. Here we demonstrate a synthetic pathway for the production of acetone through the mevalonate precursor hydroxymethylglutaryl-CoA. Expression of this pathway in various strains of Escherichia coli resulted in the co-production of acetone and ethanol. Metabolic engineering and control of the environmental conditions for microbial growth resulted in controllable acetone and ethanol production with ethanol:acetone molar ratios ranging from 0.7:1 to 10.0:1. Specifically, use of gluconic acid as a substrate increased production of acetone and balanced the redox state of the system, predictively reducing the molar ethanol:acetone ratio. Increases in ethanol production and the molar ethanol:acetone ratio were achieved by co-expression of the aldehyde/alcohol dehydrogenase (AdhE) from E. coli MG1655 and by co-expression of pyruvate decarboxylase (Pdc) and alcohol dehydrogenase (AdhB) from Z. mobilis. Controlling the fermentation aeration rate and pH in a bioreactor raised the acetone titer to 5.1 g L(-1) , similar to that obtained with wild-type Clostridium acetobutylicum. Optimizing the metabolic pathway, the selection of host strain, and the physiological conditions employed for host growth together improved acetone titers over 35-fold (0.14-5.1 g/L). Finally, chemical catalysis was used to upgrade the co-produced ethanol and acetone at both low and high molar ratios to higher-chain oxygenates for gasoline and jet fuel applications. Biotechnol. Bioeng. 2016;113: 2079-2087. © 2016 Wiley Periodicals, Inc.

Engineering Clostridium acetobutylicum for production of kerosene and diesel blendstock precursors.

Processes for the biotechnological production of kerosene and diesel blendstocks are often economically unattractive due to low yields and product titers. Recently, Clostridium acetobutylicum fermentation products acetone, butanol, and ethanol (ABE) were shown to serve as precursors for catalytic upgrading to higher chain-length molecules that can be used as fuel substitutes. To produce suitable kerosene and diesel blendstocks, the butanol:acetone ratio of fermentation products needs to be increased to 2-2.5:1, while ethanol production is minimized. Here we show that the overexpression of selected proteins changes the ratio of ABE products relative to the wild type ATCC 824 strain. Overexpression of the native alcohol/aldehyde dehydrogenase (AAD) has been reported to primarily increase ethanol formation in C. acetobutylicum. We found that overexpression of the AAD(D485G) variant increased ethanol titers by 294%. Catalytic upgrading of the 824(aad(D485G)) ABE products resulted in a blend with nearly 50wt%≤C9 products, which are unsuitable for diesel. To selectively increase butanol production, C. beijerinckii aldehyde dehydrogenase and C. ljungdhalii butanol dehydrogenase were co-expressed (strain designate 824(Cb ald-Cl bdh)), which increased butanol titers by 27% to 16.9gL(-1) while acetone and ethanol titers remained essentially unaffected. The solvent ratio from 824(Cb ald-Cl bdh) resulted in more than 80wt% of catalysis products having a carbon chain length≥C11 which amounts to 9.8gL(-1) of products suitable as kerosene or diesel blendstock based on fermentation volume. To further increase solvent production, we investigated expression of both native and heterologous chaperones in C. acetobutylicum. Expression of a heat shock protein (HSP33) from Bacillus psychrosaccharolyticus increased the total solvent titer by 22%. Co-expression of HSP33 and aldehyde/butanol dehydrogenases further increased ABE formation as well as acetone and butanol yields. HSP33 was identified as the first heterologous chaperone that significantly increases solvent titers above wild type C. acetobutylicum levels, which can be combined with metabolic engineering to further increase solvent production.

Utilization of lysozyme charge ladders to examine the effects of protein surface charge distribution on binding affinity in ion exchange systems.

A lysozyme library was employed to study the effects of protein surface modification on protein retention and to elucidate preferred protein binding orientations for cation exchange chromatography. Acetic anhydride was used as an acetylating agent to modify protein surface lysine residues. Partial acetylation of lysozyme resulted in the formation of a homologous set of modified proteins with varying charge densities and distribution. The resulting protein charge ladder was separated on a cation exchange column, and eluent fractions were subsequently analyzed using capillary zone electrophoresis and direct infusion electrospray ionization mass spectrometry. The ion exchange separation showed a significant degree of variation in the retention time of the different variants. Several fractions contained coelution of variants, some with differing net charge. In addition, several cases were observed where variants with more positive surface charge eluted from the column prior to variants with less positive charge. Enzymatic digest followed by mass spectrometry was performed to determine the sites of acetylation on the surface of the variants eluting in various fractions. Electrostatic potential maps of these variants were then generated to provide further insight into the elution order of the variants.

Production of an acetone-butanol-ethanol mixture from Clostridium acetobutylicum and its conversion to high-value biofuels.

Clostridium acetobutylicum is a bacterial species that ferments sugar to a mixture of organic solvents (acetone, butanol and ethanol). This protocol delineates a methodology to combine solventogenic clostridial fermentation and chemical catalysis via extractive fermentation for the production of biofuel blendstocks. Extractive fermentation of C. acetobutylicum is operated in fed-batch mode with a concentrated feed solution (500 grams per liter glucose and 50 grams per liter yeast extract) for 60 h, producing in excess of 40 g of solvents (acetone, butanol and ethanol) between the completely immiscible extractant and aqueous phases of the bioreactor. After distillation of the extractant phase, the acetone, butanol and ethanol mixture is upgraded to long-chain ketones over a palladium-hydrotalcite (Pd-HT) catalyst. This reaction is generally carried out in batch with a high-pressure Q-tube for 20 h at 250 °C. Following this protocol enables the production of ∼0.5 g of high-value biofuel precursors from a 1.7-g portion of fermentation solvents.

Chemocatalytic upgrading of tailored fermentation products toward biodiesel.

Biological and chemocatalytic processes are tailored in order to maximize the production of sustainable biodiesel from lignocellulosic sugar. Thus, the combination of hydrotalcite-supported copper(II) and palladium(0) catalysts with a modification of the fermentation from acetone-butanol-ethanol to isopropanol-butanol-ethanol predictably produces higher concentrations of diesel-range components in the alkylation reaction.